GTA SA lags real bad on amd radeon hd 5770?

GTA San Andreas lags on my new sapphire radeon hd 5770 1gb, I get about 10-20 fps even when the settings are set to low but it played incredibly well at maxed out settings on my old geforce 8600gt. I tried running it on another laptop with an ati radeon hd 5450 but it still lags real bad even at low settings. The hd 5770 handles pretty much any new game at medium to high settings with decent fps, why is it failing to run GTA SA?|||What are you other specs?